President-elect Barack Obama has plenty of serious problems on his agenda, but here’s another worthy one: a war on obesity.

The reason: Fatness is killing Americans by the millions. It’s driving up health costs and damaging the national economy. It’s also aesthetically disgusting.

And Obama is singularly positioned to lead the charge against overweightness: He’s lean, he exercises and he can set a great example for getting the nation fit again.

He’s also smart enough to figure out how to do it — which is probably to mention the obesity problem in his health care reform speeches and assign his yet-to-be-named surgeon general to mount a campaign of exhortation and scolding.

If it were up to me, being fat would be made as socially unacceptable as smoking. (And, before you send me an irate e-mail, as a formerly overweight person, I admit to some prejudice here.)

We can’t put fat people outside in the cold, I suppose, but the fact is that incidence of smoking has dropped from 42 percent in 1965 to below 20 percent, reducing cases of lung cancer along the way.

I like what Southwest Airlines does —charge double for people who can’t fit into one seat. I’d also be for higher health insurance premiums for fat people and for taxes on fast foods with high fat and sugar content.

New York Gov. David Paterson (D) has talked about fostering social pressure on people to lose weight, but Obama probably would prefer a kinder and gentler approach.

After showing interest in the problem — and repeating it often enough to show he’s serious — Obama can advance the cause by continuing to let photographers show him working out, playing basketball and hunking it on the beach.

The Washington Post reported last week that, whatever else he has to do, Obama works out 90 minutes a day. Pictures of him on the beach during his vacation in Hawaii show the results.

Why bother to take on this cause? Because no less than 66.3 percent of U.S. adults and nearly 20 percent of children are overweight, according to the Centers for Disease Control.

More than a third of adults — more than 72 million — are obese, fatter than fat. That percentage has doubled since 1980. The figure for children has tripled.

According to the Almanac of Chronic Disease, unless something is done, by 2015, three-quarters of U.S. adults will be overweight and 41 percent obese.

What’s fat? What’s obese? Officially, it’s measured by body mass index. For an adult 5 feet, 9 inches tall, weighing more than 169 pounds (BMI 25) is overweight and 203 pounds (BMI 30) is obese.

Obama, at 6-1 and 180 pounds, has a BMI of 23.7 — perfect.

(You can easily find out your own at the National Institutes of Health Web site, www.nhlbisupport.com/bmi/. For what it’s worth, mine was 27.4. Now it’s 24.3. It is possible to lose 20 pounds.)

The reason a president should tackle the issue of obesity is this: It kills and it costs.

Incidence of Type 2 diabetes — which can lead to blindness, loss of limbs and even mental impairment — has doubled over the past three decades on track with obesity.

There has also been an upsurge in incidence of high blood pressure, coronary heart disease, strokes and gall bladder ailments.

According to the nonprofit Partnership to Fight Chronic Disease, the doubling of obesity accounts for nearly 30 percent of the rise in health care spending since 1987.

If the prevalence of obesity was the same as it was in 1987, the group said, health care spending per capita would be 10 percent less than it is, saving $200 billion.

The burden of obesity on society is felt in Medicaid and Medicare spending and in everyone’s insurance premiums.

It would be impolite, I know, to walk up to a fat person and say: “You know, you are costing me money,” as you might complain without compunction to someone smoking.

But it’s a fact. Insurance companies are afraid of lawsuits alleging discrimination, so many of them offer premium discounts for people who enter fitness programs, but they don’t charge extra based on weight.

That means everyone else pays higher premiums to insure the overweight and pay for treatment of their diseases.

At a recent gathering of corporate chief executive officers sponsored by the Wall Street Journal, the top health care concern the group thought Obama should address was obesity and its burden on their costs.

News reports indicate that some employers refuse to hire fat people, fearing their insurance costs will balloon.

Some of these workers are being unfairly discriminated against, based on genetics or hormonal imbalance, but certainly not most.

Trimming down America probably is going to be harder during a deep recession than at other times. There’s a socioeconomic correlation to obesity — it’s more prevalent among poor people than wealthier folks.

And, depressing circumstances undoubtedly cause people to seek comfort in food and being fat probably increases depression, creating a vicious cycle.

So, it’s going to be tough right now for Obama to get people to eat less and exercise more. But it can be done and it’s definitely worth the try.